EVN and Global VLBI results and images

We present recent results and images made with the
European VLBI Network, often in conjunction with other telescopes -
predominantly MERLIN, the VLBA or VSOP. Click on the links below for
the full resolution images. In most cases a brief description of the
experiment and the significance of the result is also provided.
Links to electronic versions of associated publications are listed
where possible. We encourage PIs to contact us if they have any EVN results
that they would like to highlight on these pages.

If you would like to make use of any of these images please send an email to
Zsolt Paragi. We will then seek permission
from the author. Use of EVN images in any publication should
clearlyacknowledge
the European VLBI Network.

Location of current and future EVN stations

The European VLBI Network (EVN) is an interferometric array of radio
telescopes spread throughout Europe and beyond. The figure color-codes:

The underlying map was generated from
EarthView, specifically
this page to get
a form to specify a "satellite-view" of the planet.

The world map at left displays all current (December 2016) EVN stations, plus the correlator at JIVE. This 1800x1067 pixel image was produced using a Blue Marble Next Generation image from NASA Visible Earth. This map is also available in other sizes and with other telescope subsets. Contact Paul Boven (boven@jive.eu) for permission and more information.

The magnetic field at milliarcsecond resolution around IRAS 20126+4104O

EVN observations (project code: ES066) have been used to determine the morphology of the magnetic field on AU scales around the massive young stellar object (YSO) IRAS20126+4104 by observing the polarized emission of the 22 GHz water masers and the 6.7 GHz methanol masers. The orientation of the magnetic field derived from the masers agrees with the S-shaped morphology seen on larger scales by using the dust-polarized emission at 350 μm.

Figure left: The black bars represent the magnetic field direction in the young stellar object YSO IRAS20126+4104, determined from the polarized dust emission at 350 μm, with the continuum emission shown in grey scales in the background. The white box indicates the position of the right panel. Right: methanol (triangles), OH (squares), and water (octagons) masers in IRAS20126+4104. The gold asterisk represents the B0.5 protostar, while the dotted line represents the Keplerian disk of 1000 AU. The red and blue lines indicate the red- and blue-shifted lobes of the jet, respectively. The thick green segments represent the magnetic field direction determined from the polarized methanol and water maser emissions. The green dashed segments represent the magnetic field direction determined from the linearly polarized emission of OH masers (Edris et al. 2005, A&A, 434, 213).

Published in: Surcis, G. et al. 2014: A&A 563, 30

Polarization observations of methanol maser emission in YSOs

Despite the recognized importance of magnetic fields in the formation of low-mass stars their role in the formation of high-mass stars is still under debate. A long-term EVN program (project code: ES066; see above) has been undertaken to make polarization observations of the 6.7GHz methanol maser emission in young stellar objects (YSOs).

Further observations (project code: ES072) have been made of the magnetic field morphology towards 8 new massive YSOs. These sources are part of a large sample of massive YSOs, the so-called flux-limited sample composed of 31 sources. A preliminary statistical analysis of the results obtained from the first 19 sources reveals evidence that the magnetic fields around massive YSOs are preferentially oriented along the molecular outflow.

Figure: A view of the methanol masers features detected in the massive star forming region S231. The solid black lines represent the linear polarisation vectors of the methanol maser feature. The magnetic field is perpendicular to the linear polarisation vector.

A study has been made of high-mass star-forming regions associated with both 6.7 GHz methanol and 4.5 μm
mid-infrared (MIR) emission, which likely traces outflows from massive young stellar objects (MYSO). The objectives were to determine the milliarcsecond morphology of the maser emission and to examine if it is related to a single or several MIR counterparts in the clusters of MYSOs. EVN observations
(project code: EB043)
were carried out with near simultaneous 32 m Torun observations in a 2.1 arcmin field. Maps were obtained with mas angular resolution that showed diversity of methanol emission morphology: a linear distribution (e.g.,
G37.753-00.189), a ring-like (G40.425+00.700), and a complex one (e.g., G45.467+00.053). The maser emission is usually associated with the strongest MIR counterpart in the clusters; no maser emission was detected from other MIR sources in the EVN field of view. The maser source luminosity seems to correlate with the total luminosity of the central MYSO. Although the EVN resolves out a significant part of the maser emission, the morphology is still well determined. This indicates that the majority of maser components have compact cores.

Figure:
Example spectra and maps of 6.7 GHz methanol masers detected using the EVN. The colours of circles relate to the LSR velocities as shown in the spectra. The grey lines show the Torun 32 m dish spectra. The flux density scales are shown separately for the EVN (left) and Torun (right). The thin bars under the spectra show the LSR velocity ranges of spots displayed.

Published in: Bartkiewicz, A. et al. 2014: A&A 564, 110

A multiple system of high-mass YSOs surrounded by disks in NGC 7538 IRS1

The EVN has been used to observe NGC7538 IRS1 (at 2.7 kpc) in the 6.7 GHz maser line of methanol, which is a typical signpost of high-mass star formation. Four observing epochs spanning eight years
(project code: ES063)
were used to measure positions, proper motions, line-of-sight
velocities and accelerations of methanol masers. These measurements provided 3D kinematics and dynamics of circumstellar
gas on scales from tens to a thousand AU. Compelling evidence is found that NGC7538 IRS1 is forming a multiple system of high-mass young-stars
surrounded by disks. Data modelling enabled identifying quasi-Keplerian
rotation around a 25 M☉
star and a thick disk containing 16 M☉
around a nearby protostar. These measurements are critical to test theoretical models of accretion for massive stars.

Figure:
The NGC7538 Star Forming Region (2.7 kpc). Left: Three-colour IRAC image in the mid-infrared from the Spitzer Space Observatory. Right: 6.7 GHz methanol maser emission spots (filled circles) observed with the EVN overlaid on the 22 GHz continuum map imaged with the VLA (black image and white contours). Colors denote line-of-sight velocities, with blue indicating blue-shifted emission and red indicating red-shifted emission. The sizes of the circles scale with the flux density. The ellipses represent the disk planes surrounding the two young stars IRS1a and IRS1b, resolved by the EVN, with the solid lines indicating the near-side and the dashed lines the far-side of the disks.

Published in: Moscadelli, L. & Goddi, C. 2014: A&A 566, A150

EVN observations of the γ-ray flare in the classical nova V959 Mon

On 19th June 2012 the Fermi γ-ray Space Telescope detected emission from the classical nova V959 Mon, lasting for 12 days. EVN e-VLBI Target of Opportunity observations (project codes: RO005 and RO006) were made in September and October 2012. It was found that the ejecta from this nova are shaped by the motion of the binary system; some gas is being expelled rapidly along the poles as a wind from the white dwarf, while denser material has drifted out along the equatorial plane, propelled by orbital motion. At the interface between the equatorial and polar regions the observed synchrotron emission is indicative of shocks and relativistic particle acceleration, thereby also pinpointing the location of the γ-ray production. Binary shaping of the nova ejecta and associated internal shocks are expected to be widespread among novae, explaining why many novae are γ-ray emitters.

Figure:
Radio imaging of V959 Mon. (a) The EVN images at age 91 days (contour lines) and 113 days (in colour) after the γ-ray discovery. These images show the compact radio knots expanding diagonally. (b) e-MERLIN image of the ejecta from the nova explosion (in colour) from 87 days after discovery with the EVN contours from day 91 superimposed. (c) The same components as (b) but this time imaged with the VLA 126 days after discovery (in colour) and EVN at day 113 (contours). (d) VLA image after 615 days (in colour) with contours from the day 16 image which shows how the nova ejecta has expanded and how the major axis of the radio emission flips from E-W (horizontal in image c) to N-S. Scale bars assume a distance of 1.5 kpc and the white cross is the presumed location of the central binary system.

PSR J0218+4232 is a millisecond pulsar (MSP) with a flux density of 0.9 mJy at 1.4 GHz. It is very bright in the high-energy X-ray and γ-ray domains. An astrometric program using the EVN at 1.6 GHz
(project code: EY015)
has been conducted to measure its proper motion and parallax. A model-independent distance would help constrain the pulsar's γ-ray luminosity.
Detections of the MSP were achieved with signal-to-noise ratios of at least 37 in all 5 epochs. The EVN-derived proper motion value has significantly improved upon those arising from long-term pulsar-timing observations. The EVN parallax determination was 0.16 +/- 0.09 mas.
This was the first trigonometric parallax measurement based soley on EVN observations for any pulsar. This parallax also provided the first model-independent distance estimate for this pulsar, with a corresponding 3-sigma lower-limit of d = 2.3 kpc.
The derived distance suggests that PSR J0218+4332 may be among the most energetic γ-ray MSPs known to date. The high observed luminosity could pose challenges to the conventional outer–gap and slot-gap models.

Figure:
Astrometric fit to the motion of the milli-second pulsar J0218+4232 as observed by the EVN. The ellipses show the positional uncertainty at each epoch, the dotted line represents the modeled motion on the sky, and the star symbols mark the modeled position at each epoch. The left panel includes only proper motion in the model, while the right panel also includes parallax.

Published in: Du, Y. et al. 2014: ApJ 782, L38

Precision astrometry of pulsars and other compact radio sources in M15

M15 is a massive globular cluster that is known to contain some pulsars and a radio-loud low-mass X-ray binary. Multi-epoch astrometric EVN observations have been conducted
(project code: GV020)
to trace the kinematics and variability of compact radio sources in M15. Proper motions of two pulsars, including the double neutron star system M15C, and the LMXB were determined. Two previously unknown compact sources were detected, whose kinematics point to their not being in M15. Brightness variability was seen in M15C (disappearing in epochs 5-6 and re-appearing in epoch 7, accompanied
by a slight offset in the pulse phase), which may be a consequence of geodetic precession, as the spin axis of the "visible"
neutron star shifts due to coupling with its partner, initially moving the beamed emission out of our line of sight and later returning a different component of the emission cone back into it. The LMXB brightened by a factor of about 2.5 in the third epoch and showed a double-lobe
structure with a separation on the order of 140 AU, suggesting that it underwent an outburst during the three months between epochs 2-3 (and recovered before epoch 4).
Finally, the lack of central faint emission after stacking all epochs (rms about 3.3 μJy/beam), together with the "fundamental plane"
of black hole activity relating radio and X-ray luminosity to mass, places an upper limit to the mass of a putative intermediate-mass black hole in the center of M15 of < 500 M☉

Figure:
none

Published in: Kirsten, F. et al. 2014: A&A 565, A43

The annual parallax of cataclysmic binary system AM Herculis using e-VLBI

AMHer is the prototype, and first discovered member, of the class of polars. These are a type of cataclysmic variable where the white dwarf magnetic field is strong enough to direct the flow of accretion onto its magnetic pole(s). In order to make a new, more precise distance estimate for AM Her, a new astrometric survey using EVN e-VLBI at 5 GHz was conducted. Multi-epoch phase-referencing observations
(project code: EG069)
started in December 2012 and were performed during a 12-month period. AM Her was detected at all epochs of observation. With the addition of two archival VLA observations, a new parallax estimate (π = 11.30+/-0.35 mas, d = 88.5+/-2.8 pc) was made.

Figure:
A comparison of the EG069 astrometric measurements with the model of the annual parallax and proper motion. The figure shows position offsets of AM Her relative to the first epoch observations.

Published in: Rycyk, G. et al. 2014: PoS (EVN 2014) 106

Pulsar scattering in space and time

Scattering by plasma density fluctuations in the interstellar medium has two effects that can be studied with radio-astronomical observations. (i) Strong scattering produces many sub-images (or speckles) by effectively deflecting the radiation along different paths in the "scattering screen". These images cannot be resolved with standard techniques but appear as angular scatter-broadening and can be quantified with VLBI. (ii) The different paths correspond to different travel times, so that signals will also be spread in time, which can be detected in the case of pulsars as (temporal) pulse broadening. The ratio of the strength of both effects depends on distances and can thus be used to determine the location of the scattering screen, provided that the assumption of having only one thin screen is valid.

A new approach applied in a recent global VLBI experiment compares the two effects not only after integrating over all sub-images, but tries to measure the scattering delay as a function of the deflection angle (or vice versa). For a given delay (seen as part of the scattered pulse profile) a narrow ring should be seen, whose size grows with the square root of the delay. In this way it can be tested if the thin-screen scenario, on which this expectation is based, is valid, or if more complicated geometries have to be considered.

A number of pulsars were observed with five EVN stations, the VLBA and Arecibo (project code: GW022).
A number of correlation passes with DiFX were necessary to first produce a data set integrated over the pulse profile (weighted to maximise SNR) as basis for the phase calibration. The same calibration was then applied to binned correlations, i.e. many data sets for a number of bins within the pulse profile, corresponding to a range of scattering delays.

Early results for one pulsar show that the observed scattered pulse profile does indeed depend on the baseline length. Short baselines (large scales) show the widest profiles, which include the longest delays. On longer baselines the profile becomes narrower because only the inner parts of the scattering disk with the smallest deflection angles (and smaller delays) can be seen. It is even found that on longer baselines the profile (as a visibility) becomes negative for certain delay ranges, which is expected for ring-like structures that grow with delay. A more continuous distribution of scattering material between the pulsar and the observer would instead produce growing circular Gaussian structures without negative visibilities. Such models can be ruled out already, but more detailed conclusions have to wait for further analysis.

Figure:
Pulse profiles of B1815-14 observed at 1.4 GHz (left) in autocorrelation (Effelsberg) and (centre and right) on 2 baselines with resolutions 0.621 kλ and 0.918 kλ, for one scan of about 15 min. The real part is shown in solid blue, the imaginary part in dotted red. The negative pre-pulse dip in the left plot results from digitisation non-linearities and dispersion.

Published in: Wucknitz, 0. 2013: PoS (11th EVN Symposium) 049

The NGC 3341 minor merger: a panchromatic view of the AGN in a dwarf companion

NGC 3341 is a nearby (z = 0.027) disturbed system consisting of a minor merger between a giant disc galaxy (MB = 20.3 mag) and two low-mass dwarf companions within the galaxy disc. While spectroscopic data confirmed the physical association between the three objects, an investigation on the presence of supermassive black holes and of associated nuclear activity was missing. In combination with SDSS, Chandra and EVLA data, EVN e-VLBI observations at 1.6 and 5 GHz (project code: RSP008) were made to explore the nature of the components of NGC 3341. The high angular resolution provided by the EVN resolves out the radio emission seen with the VLA, favouring a multi-wavelength scenario in which the nucleus is a star-forming region.

Figure left:
optical (SDSS i filter); Center: X-rays (Chandra, 0.2-8 kev); Right: radio (EVLA, 5 GHz). The EVN data resolve out the radio emission down to a level of 21 μJy beam-1 at 5 GHz.

Published in: Bianchi, S. et al. 2013: MNRAS 435, 2335

Constraints on the progenitor system and the environs of SN 2014J from deep radio observations

Deep EVN e-VLBI (project codes: RP023, EP092) and e-MERLIN observations have been made of the Type Ia SN 2014J in the nearby galaxy M82. These observations, along with JVLA observations of SN 2011fe, are among the most sensitive radio studies of Type Ia supernovae. By combining data and a proper modeling of the radio emission, the mass-loss rate from the progenitor system of SN 2014J has been constrained to < 7.0 x 10−10M☉ per year. Assuming that the medium around the supernova is uniform, then the density of the ISM is less than 1.3 cm−3, which is the most stringent limit for the (uniform) density around a Type Ia supernova. These deep upper limits favour a double-degenerate (DD) scenario involving two white dwarf (WD) stars for the progenitor system of SN 2014J, as such systems have less circumstellar gas than these upper limits. By contrast, most single-degenerate (SD) scenarios, i.e., the wide family of progenitor systems where a red giant, main-sequence, or sub-giant star donates mass to an exploding WD, were ruled out by the observations. Although a SD scenario might pass observational tests, and there are uncertainties in the modeling of the radio emission, the evidence from SNe 2011fe and 2014J points in the direction of a DD scenario for both.

Figure left:
EVN e-VLBI and e-Merlin images of the region surrounding SN 2014J, and an optical image of the host galaxy M82. Right: Constraints on the parameter space (wind speed vs. mass-loss rate) for single degenerate scenarios in the case of SN2014J. Mass-loss scenarios falling into the grey shaded areas should have been detected by the deep radio observations, and therefore are ruled out for SN 2014J. A reassessed limit for SN 2011fe (dash-dotted line) for the same choice of parameters as the solid line for SN 2014J is also shown for comparison. These constraints leave room only for quiescent nova emission as a viable alternative among the SD scenarios for SN 2011fe.

Published in: Pérez-Torres, M. et al. 2014: ApJ 792, 38

Black hole lightning due to particle acceleration at sub-horizon scales

IC 310 is a peculiar radio galaxy located in the outskirts of the Perseus cluster at z = 0.189. In November 2012 MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) revealed an outburst of very high energy (VHE) γ-rays from this object, characterized by unprecedented brightness and rapidity. Near-simultaneous EVN data (project code: EE009) greatly help to constrain the allowed ranges of orientation and velocity parameters. With these constraints the γ-ray emission region size and the possible particle acceleration mechanisms have been determined. This could be pulsar-like, with acceleration by the electric field across a magnetospheric gap at the base of the radio jet. These results are unprecedented in the field of particle acceleration in relativistic jets, and they have been the subject of several press releases throughout Europe.

Background image:
significance map of the Perseus cluster in γ-rays observed on the night of 12/13 November 2012 with the MAGIC telescopes. Inset: EVN radio jet image of IC 310 at 5.0 GHz obtained on 29 October 2012. Contour lines (and the colour scale associated with them) increase logarithmically by factors of 2, starting at 3 times the noise level.

Published in: Aleksi&cacute;, J. et al. 2014: Science 346, 1080

Things that go bump in the night: the curious case of NGC 660

An analysis has been made of observations of the LINER galaxy NGC 660. NGC 660 was found to have increased in radio flux over the period 2008-2013. Observations with the EVN (project codes: EA054, EA055) and with e-MERLIN were made to investigate the source at the smallest possible scales. A core-jet structure is detected in the nucleus, providing evidence for (re-)started nuclear activity. From an in-depth analysis of archival observations at a range of frequencies and resolutions it is confirmed that this is a new radio source, with an age of at most a decade.

EVN observations of compact radio emission in extragalactic ultraluminous X-ray sources (ULX) have been combined with data from the HST, Chandra, and the VLA in order to understand better the nature of these enigmatic objects. Most of the scenarios proposed to explain the ULX assume that they are powered by accretion on to a black hole (BH), with the expected BH masses ranging from 10 to 104M☉. The upper bound of the BH mass signifies the elusive class of intermediate mass black holes (IMBH) believed to be remnants from the early epochs of BH formation in the Universe. VLBI detections of compact radio emission in the ULX provide accurate locations of these objects within their host galaxies and allow estimates of the ULX BH mass to be made and searching for the IMBH in galaxies.

Dedicated EVN and VLA studies of 12 ULX objects performed at 1.6 and 5 GHz have provided strong evidence for the presence of IMBH in at least 2 of the systems studied (N5457-X9 and N2276-3c). One of these (N4449-X1) is clearly associated with a supernova remnant. Upper limits on compact radio emission have been placed in the remaining 9 ULX.

The EVN observations at 1.6 GHz (project code: EM095) of N5457-X9 reveal compact radio emission with a flux density of 0.12 mJy and brightness temperature TB > 8 x 104 K, coincident with the Chandra X-ray position within 0.6 arcsec. The compact source has a physical size of about 1 pc, suggesting that there are 2 radio components present in the area around this source: an extended thermal component, as revealed by single-dish and VLA radio observations, and a compact non-thermal component. Combining the radio detection with the Chandra and Swift measurements yields a BH mass in the range of 106 - 107M☉.

Figure:
Optical DSS image of the galaxy NGC 5457 and the location of the ULX source N5457-X9 detected with Chandra (marked by the Chandra error circle for this object). The inset shows a 1.6 GHz EVN image of ULX N5457-X9 made at the Chandra position. The contours are (−3, 3, 3.5, 4, 4.5, 5 and 5.2) times the off-source rms noise of 19 μJybeam−1. The restoring beam size is 30 x 20 mas, with the major axis oriented along a position angle of −11°.

Published in: Mezcua, M. et al. 2013: MNRAS 436, 1546

Resolved radio structure of a supernova remnant in NGC 4449

1.6 GHz EVN observations of the supernova remnant SNR 4449-1 (project code: EM072), made as part of an investigation of radio counterparts of ultraluminous X-ray sources, have revealed the intricate structure of the evolved radio emission in the remnant. The observations confirmed earlier identifications of this object with a supernova remnant (SNR). The 1.6 GHz image yields accurate estimates of the size (0.0422 × 0.0285 arcsec or 0.8 × 0.5 pc) and age (about 55 yr) of SNR 4449-1. With a total flux of 6.1 ± 0.6 mJy measured in the EVN image, the historical lightcurve of the source can be well represented by a power-law decay with a power index of −1.19 ± 0.07. The SNR exhibits a decline rate of the radio emission of 2.2 ± 0.1% yr−1 and a radio luminosity of 1.74 × 1035 erg s−1.

Figure:
Resolved radio structure of the SNR in NGC 4449 using the EVN at 1.6 GHz. The rms noise off-source is 42 μJy beam−1. Contours start at −1.5 times the rms and increase with factors of √2. The six components detected are labeled A, B, C, D, E, and F. The brightness peak of the map corresponds to component E and is 0.287 mJy beam−1. The dimensions (full width at half maximum) of the restoring beam are 7.9 × 3.5 mas, with the major axis of the beam oriented along a position angle of 25.8°. North is up and East is to the left.

Published in: Mezcua, M. et al. 2013: MNRAS 436, 2454

Radio jets clearing the way through a galaxy: watching feedback in action

An analysis of global HI spectral-line VLBI data on 4C 12.50 (project code: GM062) has been made. The goal was to study the spatial distribution of the outflowing natural gas in this young AGN, which is one of the best known ultra-luminous infrared galaxy (ULIRG). The neutral gas has a strong feedback on the ISM, and this feedback plays a key role in galaxy evolution. A previously known HI absorption feature in the direction of the northern counter-jet was detected, as well as a broad (about 1000 km/s in velocity) HI component that was not seen at 10 mas resolutions before; the latter coincided with the termination point of the southern, approaching jet. A significant part of the absorption comes from a compact cloud that is seen in projection to be co-spatial with the hot-spot observed earlier in radio continuum. There is also a faint and diffuse component that extends at least 50 pc around and in front of the southern lobe. These observations showed that the radio plasma drives the outflow and removes gas from the central regions in 4C 12.50, and that jet-driven outflows can play a relevant role in feedback mechanisms in radio-loud AGN.

Figure:
Total intensity (contours) and HI absorption (colours) in the approaching southern jet-lobe (left) and on the counter-jet side (right) of 4C 12.50. The inset in the middle shows the HI (orange) and CO (from mm observations by another group, in grey) absorption profiles.

Published in: Morganti, R. et al. 2013: Science 341, 1082

Sub-parsec radio cores in nearby Seyfert galaxies

As the sensitivity of radio telescopes has improved, it has become increasingly clear that many radio quiet (RQ) AGNs are not radio silent at all. However, the origin of radio emission in RQ AGNs still remains unclear and it could be ascribed to a low-power jet, to free-free emission from a molecular torus, a disc wind or the X-ray corona itself. Building on previous work the first systematic study at VLBI spatial resolutions of a complete sample of RQ local Seyfert galaxies has been made, with the purpose of characterizing statistically the physical properties of the sub-pc cores and the incidence of associated jet/outflow structures. In this work, sensitive EVN observations (project codes: EG037, EG040) have provided a fundamental resource to establish a detection rate of sub-pc scale cores as high as about 75%, with varied morphology and interesting distributions of brightness temperatures, spectral indexes, radio-to-X-ray flux ratios and radio-to-BH mass. While more observations will certainly be useful to clarify the details of the overall picture, the current results already point to a significant presence of jets and outflowing features, and to a possible dichotomy between type 1 and type 2 AGNs in terms of emission mechanisms, with free-free absorption more important in the latter.

An analysis has been made of archival EVN and MERLIN data (program code: EB019) of the core of the galaxy NGC 4418, which is one of the most obscured luminous infrared galaxies (LIRGs) in the nearby Universe. This nucleus contains a rich molecular gas environment, has an unusually high ratio of infrared-to-radio luminosity, and is either powered by a compact starburst or an active galactic nucleus (AGN). The VLBI-scale nuclear radio structure of NGC 4418 was found to consist of eight compact (< 49 mas, i.e. < 8 pc) features spread within a region of 250 mas, (41 pc). An inverted spectral index α ≥ 0.7 (Sν ∝ να) was fitted to the compact radio emission. The complex morphology and inverted spectrum of the detected compact features is evidence against the hypothesis that an AGN alone is powering the nucleus of NGC 4418. The compact features could be super star clusters with intense star formation, and their associated free-free absorption could then naturally explain both their inverted radio spectrum and the low radio-to-IR ratio of the nucleus. In order to power the radio emission via electrons accelerated in supernovae the required star formation rate per unit area is extreme. This star formation density is close to the observational limit expected for a well-mixed thermal/non-thermal plasma and is also close to the limit of what can be physically sustained dynamically.

Figure
The EVN and MERLIN 5GHz image of NGC 4418. There are eight compact features labelled from A to H. Label S corresponds to the published 860 μm continuum peak position. The rms noise is 90 μJy/beam and the clean beam is 20.6 × 14.8 mas.

Published in: Varenius, E. et al. 2014: A&A 566, 15

M87 radio brightening

A monitoring project on the radio galaxy M87 (project codes: EG063, EH027) has continued, following the success of previous years. This monitoring is carried out in collaboration with other radio arrays, such as the VLBI Exploration of Radio Astrometry (VERA) in Japan, and high and very-high energy observations by instruments such as the Fermi γ-ray satellite and Very High Energy (VHE) imaging atmospheric Cherenkov telescopes. The main aim is the determination of the site of high energy emission in the M87 jet which, thanks to its proximity, is an ideal laboratory to study relativistic jets in AGNs in general. An enhancement of the VHE flux took place in 2012 and the VERA and EVN data indicate that during this episode the most likely location for the VHE flare was the radio core, rather than the jet feature HST-1. The monitoring has further continued and new, exciting results are expected in the coming years.

Figure:
VLBI images of the M87 jet during the elevated VHE γ-ray state in 2012 March. The main (background) image was obtained with the EVN at 5 GHz. The bottom right inset indicates a close-up view toward the HST-1 region. The upper-left two insets show VERA images for the core at 22 and 43 GHz. For the EVN images, circular Gaussian beams of 4.5 and 10 mas are used for the main structure, and the HST-1 close up, respectively.

Published in: Hada, K. et al. 2014: ApJ 788, 165

The high-z flaring blazar TXS 0536+145

The radio source TXS 0536+145 is associated with a flat spectrum radio quasar at redshift 2.69. On 2012 March 22 it was observed during a γ-ray flare by the Large Area Telescope on board the Fermi satellite. The flaring episode triggered a multi-wavelength campaign, from radio to X-rays, aimed at confirming the association of the γ-ray source with the low-energy counterpart. The observed multiband variability allowed a secure identification of the γ-ray source with the high-z blazar TXS 0536+145 to be made, which becomes the γ-ray flaring blazar at the highest redshift observed so far, with an apparent isotropic peak luminosity of 6.6 × 1049 erg s−1.

EVN high angular resolution observations at 22 GHz (project code RO004) were able to resolve the initial part of the jet at 0.5 mas from the core. The radio light curve obtained on the basis of multi-epoch observations shows an increase of the flux density about 4 months after the γ-ray flare, suggesting the presence of opacity effects. The flux and spectral variability is observed only in the core region, while no significant variability is present in the jet component. The core spectrum turned out to be inverted when the flux density reached the maximum in the radio band. Similarly, in γ-rays a harder-when-brighter behaviour was observed. Despite the harder spectrum, no significant emission above 10 GeV is observed. At the redshift of TXS 0536+145 the flux attenuation due to the Extragalactic Background Light (EBL) should be observable below 10 GeV, leading to a curvature of the spectrum. However, due to the poor statistics it is not possible to test if the observed curvature is related to the EBL attenuation, to the Klein-Nishina effect, or if it is intrinsic to the spectrum of the source.

Figure:
EVN image of TXS 0536+145 at 22 GHz. The image is restored with a circular Gaussian of FWHM=0.5mas, plotted in the bottom-left corner. The peak brightness is 360.1 mJy beam−1 and the first contour level 1.7 mJy beam−1, which corresponds to 3 times the off-source noise level. Contour levels increase by factors of 2.

Published in: Orienti, M. et al. 2014: MNRAS 444, 3040

The parsec-scale structure of GPS radio sources

GHz-peaked spectrum (GPS) radio sources are thought to represent an early stage in the individual radio source evolution. In these objects the radio emission is totally confined within the innermost region of the host galaxy where jet-cloud interaction may take place as suggested by the high fraction of asymmetric GPS sources. The high resolution of Global VLBI (project code: GD001) allowed the morphology of 10 sources from a GPS sample to be determined. The combination of these data and earlier 1.7 GHz observations allows the spectral index distribution across the source structure to be studied, and an unambiguous determination of the nature of each component to be made. In 7 sources core components with a flat or inverted spectrum were detected. In six sources the radio emission has a two-sided morphology and comes mainly from steep-spectrum extended structures, like lobes, jets and hotspots. In three out of the six sources with a two-sided structure the flux density arising from the lobes is asymmetric, and the brightest lobe is the one closest to the core, suggesting that the jets are expanding in an inhomogeneous ambient medium which may influence the source growth. The interaction between the jet and the environment may slow down the source expansion and enhance the luminosity due to severe radiative losses, likely producing an excess of CSS radio sources in flux density limited samples.

Figure:
0428+205 global VLBI image at 5 GHz. The restoring beam is 1.9 mas × 0.75 mas in p.a. −15°, and is plotted
in the bottom left corner; the peak brightness is 155.5 mJy beam−1. The first contour level is 1.7 mJy beam−1, which is three times the off-source noise level; contour levels increase by factors of 2. From the spectral analysis, it is suggested that the core, C, is the northern component, which accounts for 29 mJy (i.e. 1.2% of the total VLA flux density), while the elongated component J is likely the main jet. The southern lobe, S, is dominated by a compact component, likely the hotspot, located about 50 mas (about 175 pc) from the core.

Published in: Dallacasa, D. et al. 2013: MNRAS 433, 147

EVN observations of BAL quasars

Broad absorption line quasars (BAL QSOs) are objects presenting hints of fast outflows in their UV/optical spectrum, manifested as broad absorption features in the blue-wing of emission lines such as CIV and MGII. They constitute about 15% of the QSO population, and can be even more rare among radio-loud QSOs. Two main scenarios have been proposed to explain the BAL phenomenon, one considering orientation as a key factor to account for the observed BAL QSO fraction, and another proposing a youth scenario in which the QSO is still expelling a shell of dust, which would be responsible for the absorption features.

The radio-loud brightest cluster galaxy (BCG) at the center of the cool core cluster RBS 797 is known to exhibit a misalignment of its 5 GHz radio emission observed at different VLA resolutions, with the innermost kpc-scale jets being almost orthogonal to the radio lobes which extend for tens of kpc filling the X-ray cavities seen by Chandra. The different radio directions may be caused by rapid jet reorientation due to interaction with a secondary supermassive black hole (SMBH), or to the presence of two AGN, probably in a merging phase, which are emitting radio jets in different directions.

Since no information on the pc-scale radio properties is available in the literature, an explorative program was performed to assess the detectability of the BCG in RBS 797 at VLBI resolution and to study its nuclear region. A test observation was made on 3 May, 2013 (project code RSG05) in a 6 cm e-VLBI run with a subset of the EVN. The total time spent on the target was about 1 hour. As is evident from the 5 GHz EVN map, two compact components were clearly detected. The results from a visibility model-fit with two Gaussian components indicate that both components are compact and smaller than the observing beam. There are two possible scenarios for the origin and nature of the EVN double source, and both interpretations are consistent with the presence of a SMBH binary system.

Figure left:
The 5 GHz VLA contours of RBS 797, imaged at different resolutions, are overlaid onto the Chandra image of the central region of the cluster. Green contours: 5 GHz VLA map at 1.4 × 1.3 arcsec resolution; the rms noise is 0.01 mJy beam−1 and the contour levels start at 3σ, increasing by factors of 2. Black contours: see caption centre panel. Centre: 5 GHz VLA map at 0.5 × 0.4 arcsecond resolution (the beam is shown in the lower-left corner). The rms noise is 0.01 mJy beam−1 and the contour levels start at 0.04 mJy beam−1, increasing by factors of 2. Right: 5 GHz EVN map of the BCG in RBS 797 at a resolution of 9.4 × 5.3 mas2 in P.A. 24° (the beam is shown to the lower-left). The rms noise is 36 μJy beam−1 and the peak flux density is 0.53 mJy beam−1. The contours levels start at 3σ and increase by factors of 2.

Finding binary, and even multiple supermassive black holes is of great importance because these systems have played an important role in forming galaxies in the early Universe, and they are a potential source of gravitational wave radiation as well. The EVN is particularly well suited for this quest, since its unprecedented angular resolution allows us to resolve the closest pairs of SMBH candidates. The detection of a small-separation compact double source within the system J1502+1115 has now been reported, from observations with the EVN (project codes: ED035, ED039). This system was already known to host a wider separation dual-SMBH. The compact structures and the observed flat spectra pointed to the presence of an inner dual active galactic nucleus (AGN) in a rare type of triple-SMBH system (there are only a handful of suspected candidates known), with a separation of only 140 parsec for the inner pair. This finding needs to be confirmed by further observations, because another group claimed that the VLBI structure might instead indicate a peculiar Compact Symmetric Object (CSO), a pair of compact radio lobes from a single, young active galactic nucleus. In any case, J1502+1115 remains a high-profile target for sensitive VLBI observations. The orbital motion of very small separation binary black holes may be imprinted onto their large-scale jets, twisting them into a helical or corkscrew-like shape. So even though black holes may be so close together that our telescopes cannot resolve them, their twisted jets may help locating these systems with future very sensitive instruments like the Square Kilometre Array.

Figure left:
EVN 1.7 GHz image (contours) overlaid on the EVN 5 GHz image (colour scale) of the close pair of candidate supermassive black holes in J1502+1115. Right: a sketch of spiral jets from a close pair of black holes in a triple SMBH system.

Published in: Deane, R. et al. 2014: Nature 551, 57

VLBI observations of J2228+0110 and other field sources in multiple-phase-centre mode

A patch of sky in the SDSS Stripe 82 region has been observed at 1.6 GHz using the EVN (project code: EG057). This was one of the early EVN science projects to fully exploit the multi-phase-centre capability of the SFXC software correlator. There are 15 known mJy/sub-mJy radio sources in the target field defined by the primary beam size of a typical 30 m-class EVN radio telescope. The source of particular interest was a recently identified high-redshift radio quasar: J222843.54+011032.2 (J2228+0110) at a redshift of z = 5.95. The aim was to investigate the mas-scale properties of all the VLBI-detectable sources within this primary beam area with a diameter of 20 arcmin. The source J2228+0110 was detected with a brightness temperature Tb > 108 K, as expected for a high-redshift radio-loud AGN. In addition, two other target sources were also detected, one of them with no redshift information. Their brightness temperature values (Tb > 107 K) suggest a non-thermal synchrotron radiation origin for their radio emission. The detection rate of 20% is broadly consistent with other wide-field VLBI experiments carried out recently. Accurate equatorial coordinates of the three detected sources were also derived using the phase-referencing technique.

Figure:
EVN detection of the high-redshift quasar J2228+0110 and another 2 sources with the multi-phase-centre correlation technique. The label "P-centre" marks the pointing centre of the 9 smaller telescopes working in the in-beam mode and the primary phase centre. The source J2229+0114 marked with a filled square served as the in-beam phase calibrator. The large circle is the primary beam size (FWHM) of a 32 m antenna at 1.6 GHz.